This invention relates to an improved method and device for cooling and freezing a food or other item which is carried through the device on a belt or other moving substrate. More specifically, this invention relates to a device wherein a liquid and gas-phase cryogens are used to cool and/or freeze food items that are moved through the device on a belt. The transfer of heat from the food item to the cryogen is maximized through the use of a process in which liquid cryogen is sprayed into a stream of gaseous cryogen which is circulated around the food item while also using a novel impingement plate to create a stream of cryogen. A novel design of the device increases the heat transferred from the food items to the cryogen.
Commercial freezers typically rely on the transfer of heat from a food product that is to be chilled or frozen by using a fan or blower which is situated near a conveyor upon which the food is being carried. The food product entering the freezer has a boundary layer of air surrounding it which insulates the food product in the surrounding atmosphere. Traditional freezers have employed blowers that generate currents of cooling vapor in many directions so that a significant portion of the vapor does not contact the food product in a perpendicular direction. Under these conditions the vapor which does contact the food product often does not possess sufficient energy to substantially reduce the boundary layer around the surface of the food product. There is, therefore, a need to generate directed jets of cooling vapor so as to disturb the boundary layer.
U.S. Pat. No. 4,479,776 to Smith discloses an apparatus using a plurality of vertical tubes to provide a unidirectional air flow toward the food product.
U.S. Pat. No. 4,626,661 to Henke discloses the use of a plurality of nozzles along the pathway of a food product for delivering discrete jets of unidirectional cooling air.
The use of tubes or slots to direct air in a cooling or freezing device has met with only limited success due to the build-up of condensation or ice in the tubes or slots which quickly reduces the efficacy of the devices.
U.S. Pat. No. 5,487,908 to Appolonia et al. discloses a method and device for heating or cooling a food product on a moving substrate in which a continuous channel traversing at least a major portion of the width of the moving substrate converts multi-directional flow into unidirectional flow. Such a device suffers, however, from having such an increased rate of flow that the food products become entrained in the flow and controlled processing of the food item through the device becomes difficult.
Increasing the velocity of the stream of cryogenic vapor which impinges the food item will increase the average heat transfer coefficient in a linear manner. At a certain point, however, unless the impingement stream is carefully controlled the velocity may also be sufficient to damage the food product or to carry the food product off the conveyor and into undesirable locations elsewhere in the freezer.
Overall heat transfer rates are dependent on local heat transfer coefficients, i.e., the amount of heat transferred from the food products to the cryogen is dependent on the rate of heat transfer locally between the cryogen and the food item. Increasing the local heat transfer rate by controlling the distance from the source of impingement jets to the food product, the velocity of the impingement jets, the turbulence in the jet and the efficiency of the flow of cryogen.
A need remains, therefore, for a device which can rapidly chill and/or freeze a food item while reducing the amount of cryogen needed by extracting the maximum cooling effect from a given amount of cryogen. The device must also be capable of transporting food from an inlet to an outlet without damaging the food product. Additionally, the device must be able to control the throughput of food items and must be resistant to the freezing and plugging of internal components by snow and ice build-up.